Split gate charge trapping memory cells having different select gate and memory gate heights

ABSTRACT

A semiconductor device and method of making such device is presented herein. The method includes disposing a gate layer over a dielectric layer on a substrate and further disposing a cap layer over the gate layer. A first transistor gate is defined having an initial thickness substantially equal to a combined thickness of the cap layer and the gate layer. A first doped region is formed in the substrate adjacent to the first transistor gate. The cap layer is subsequently removed and a second transistor gate is defined having a thickness substantially equal to the thickness of the gate layer. Afterwards, a second doped region is formed in the substrate adjacent to the second transistor gate. The first doped region extends deeper in the substrate than the second doped region, and a final thickness of the first transistor gate is substantially equal to the thickness of the second transistor gate.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S.Non-Provisional application Ser. No. 15/430,157, filed on Feb. 10, 2017,which claims priority to U.S. Non-Provisional application Ser. No.14/742,201, filed on Jun. 17, 2015, now U.S. Pat. No. 9,590,079, issuedon Mar. 7, 2017, which claims priority to U.S. Non-Provisionalapplication Ser. No. 13/715,673, filed on Dec. 14, 2012, all of whichare incorporated by reference herein in their entirety.

BACKGROUND Field

The present application relates to the fabrication of split-gate chargetrapping memory cells and other field-effect transistors formed in thesame substrate.

Background Art

A non-volatile memory, such as Flash memory, retains stored data even ifpower to the memory is removed. A non-volatile memory cell stores data,for example, by storing electrical charge in an electrically isolatedfloating gate or in a charge-trapping layer underlying a control gate ofa field-effect transistor (FET). The stored electrical charge controlsthe threshold of the FET, thereby controlling the memory state of thecell.

A non-volatile memory cell is programmed using, for example, hot carrierinjection to place charge into a storage layer. High drain and gatevoltages are used to facilitate the programming process, and the memorycell conducts relatively high current during programming, which can beundesirable in low voltage or low power applications.

A split-gate memory cell is a type of non-volatile memory cell, in whicha select gate is placed adjacent a memory gate. During programming of asplit-gate memory cell, the select gate is biased at a relatively lowvoltage, and only the memory gate is biased at the high voltage toprovide the vertical electric field necessary for hot-carrier injection.Since acceleration of the carriers takes place in the channel regionmostly under the select gate, the relatively low voltage on the selectgate results in more efficient carrier acceleration in the horizontaldirection compared to a conventional Flash memory cell. That makeshot-carrier injection more efficient with lower current and lower powerconsumption during programming operation. A split-gate memory cell maybe programmed using techniques other than hot-carrier injection, anddepending on the technique, any advantage over the conventional Flashmemory cell during programming operation may vary.

Fast read time is another advantage of a split-gate memory cell. Becausethe select gate is in series with the memory gate, the erased state ofthe memory gate can be near or in depletion mode (i.e., thresholdvoltage, Vt, less than zero volt). Even when the erased memory gate isin such depletion mode, the select gate in the off state prevents thechannel from conducting substantial current. With the threshold voltageof the erased state near or below zero, the threshold voltage of theprogrammed state does not need to be very high while still providing areasonable read margin between erased and programmed states.Accordingly, the voltages applied to both select gate and memory gate inread operation can be less than or equal to the supply voltage.Therefore, not having to pump the supply voltage to a higher level makesthe read operation faster.

It is common to monolithically incorporate multiple types offield-effect devices on the same substrate as the memory cells. Thosenon-memory devices perform, for example, decoding, charge-pumping, andother functions related to memory operations. The substrate may alsoinclude non-memory devices to provide functions that are not related tomemory operations. Such non-memory devices incorporated on the samesubstrate as the memory cells may include transistors tailored forhigh-speed operations, while other transistors are tailored for handlinghigh operating voltages. Integrating the processing of memory cells,such as a split-gate memory cell, with the processing of one or moretypes of non-memory transistors on the same substrate is challenging aseach requires different fabrication parameters. Accordingly, there is aneed for device and methods for integrating different types of deviceson the same substrate to facilitate improved cost, performance,reliability, or manufacturability.

SUMMARY

It is desirable to obviate or mitigate at least one of the problems,whether identified herein or elsewhere, or to provide an alternative toexisting apparatuses or methods.

According to an embodiment, there is provided an example method forfabricating a semiconductor device. The method includes disposing a gatelayer over a dielectric layer on a substrate and further disposing a caplayer over the gate layer. The method then includes etching through thecap layer and the gate layer to define a first transistor gate having aninitial thickness substantially equal to a combined thickness of the caplayer and the gate layer. Afterwards, a first doped region is formed inthe substrate adjacent to the first transistor gate. The cap layer issubsequently removed and the gate layer is etched again to define asecond transistor gate having a thickness substantially equal to thethickness of the gate layer. Afterwards, a second doped region is formedin the substrate adjacent to the second transistor gate. The first dopedregion extends deeper in the substrate than the second doped region, anda final thickness of the first transistor gate is substantially equal tothe thickness of the second transistor gate.

According to another embodiment, there is provided a semiconductordevice that includes a first transistor and a second transistor. Thefirst transistor includes a first transistor gate having a firstthickness and a first gate length, and a first doped region in thesubstrate adjacent to the first transistor gate. The second transistorincludes a second transistor gate having a second thicknesssubstantially equal to the first thickness and a second gate length lessthan half the length of the first gate length. The second transistoralso includes a second doped region in the substrate adjacent to thesecond transistor gate, wherein the first doped region extends deeper inthe substrate than the second doped region.

According to another embodiment, there is provided an example method forfabricating a semiconductor device. The method includes disposing a gatelayer over a first dielectric layer on a substrate and further disposinga cap layer over the gate layer. The method then includes forming aplurality of memory cells in a first region of the substrate. Each ofthe memory cells includes a select gate disposed over the firstdielectric, a memory gate disposed over a second dielectric and adjacentto a sidewall of the select gate, a first doped region in the substrateadjacent to one side of the select gate, and a second doped region inthe substrate adjacent to the opposite side of the memory gate. Themethod further involves etching through the cap layer and the gate layerin a second region of the substrate to define a first transistor gatehaving an initial thickness substantially equal to a thickness of thecap layer and the gate layer. A third doped region is then formed in thesubstrate adjacent to the first transistor gate. Next, the cap layer isremoved and the gate layer is etched in a third region of the substrateto define a second transistor gate having a thickness substantiallyequal to the thickness of the gate layer. Afterwards, a fourth dopedregion is formed in the substrate adjacent to the second transistorgate. The third doped region extends deeper in the substrate than thefourth doped region and a final thickness of the first transistor gateis substantially equal to the thickness of the second transistor gate.

According to another embodiment, there is provided a semiconductordevice that includes a plurality of memory cells, a plurality of firsttransistors, and a plurality of second transistors. The plurality ofmemory cells are formed in a first region of the substrate and eachinclude a select gate disposed over a first dielectric, a memory gatedisposed over a second dielectric and adjacent to a sidewall of theselect gate, a first doped region in the substrate adjacent to one sideof the select gate, and a second doped region in the substrate adjacentto an opposite side of the memory gate. The plurality of firsttransistors are formed in a second region of the substrate and eachinclude a first transistor gate having a first thickness and a firstgate length, and a third doped region in the substrate adjacent to thefirst transistor gate. The plurality of second transistors are formed ina third region of the substrate and each include a second transistorgate having a second thickness substantially equal to the firstthickness and a second gate length less than half the length of thefirst gate length. Each of the second transistors also includes a fourthdoped region in the substrate adjacent to the second transistor gate,wherein the third doped region extends deeper in the substrate than thefourth doped region.

According to another embodiment, there is provided an example method offabricating a semiconductor device. The method includes disposing a gatelayer over a first dielectric layer on a substrate and further disposinga cap layer over the gate layer. The method then includes forming aplurality of memory cells in a first region of the substrate. Each ofthe memory cells includes a memory gate disposed over the firstdielectric, a select gate disposed over a second dielectric and adjacentto a sidewall of the memory gate, a first doped region in the substrateadjacent to one side of the select gate, and a second doped region inthe substrate adjacent to the opposite side of the memory gate. Themethod further involves etching through the cap layer and the gate layerin a second region of the substrate to define a first transistor gatehaving an initial thickness substantially equal to a thickness of thecap layer and the gate layer. A third doped region is then formed in thesubstrate adjacent to the first transistor gate. Next, the cap layer isremoved and the gate layer is etched in a third region of the substrateto define a second transistor gate having a thickness substantiallyequal to the thickness of the gate layer. Afterwards, a fourth dopedregion is formed in the substrate adjacent to the second transistorgate. The third doped region extends deeper in the substrate than thefourth doped region and a final thickness of the first transistor gateis substantially equal to the thickness of the second transistor gate.

Further features and advantages of the present invention, as well as thestructure and operation of various embodiments of the present invention,are described in detail below with reference to the accompanyingdrawings. It is noted that the present invention is not limited to thespecific embodiments described herein. Such embodiments are presentedherein for illustrative purposes only. Additional embodiments will beapparent to persons skilled in the relevant art(s) based on theteachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of thepresent invention and to enable a person skilled in the relevant art(s)to make and use the present invention.

FIG. 1 illustrates a cross-section of a split-gate memory cell,according to various embodiments.

FIG. 2 illustrates connections made to a split-gate memory cell,according to various embodiments.

FIG. 3 illustrates field-effect devices formed in various regions of asubstrate, according to various embodiments.

FIGS. 4A-4H illustrate various cross-section views of a semiconductordevice fabrication process, according to embodiments.

FIG. 5 illustrates a cross-section view of field-effect devices havingdifferent characteristics, according to an embodiment.

FIGS. 6A-6F illustrate various cross-section views of a semiconductordevice fabrication process, according to embodiments.

The features and advantages of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings, in which like reference charactersidentify corresponding elements throughout. In the drawings, likereference numbers generally indicate identical, functionally similar,and/or structurally similar elements. The drawing in which an elementfirst appears is indicated by the leftmost digit(s) in the correspondingreference number.

DETAILED DESCRIPTION

This specification discloses one or more embodiments that incorporatethe features of this invention. The disclosed embodiment(s) merelyexemplify the present invention. The scope of the present invention isnot limited to the disclosed embodiment(s). The present invention isdefined by the claims appended hereto.

The embodiment(s) described, and references in the specification to “oneembodiment,” “an embodiment,” “an example embodiment,” etc., indicatethat the embodiment(s) described may include a particular feature,structure, or characteristic, but every embodiment may not necessarilyinclude the particular feature, structure, or characteristic. Moreover,such phrases are not necessarily referring to the same embodiment.Further, when a particular feature, structure, or characteristic isdescribed in connection with an embodiment, it is understood that it iswithin the knowledge of one skilled in the art to effect such feature,structure, or characteristic in connection with other embodimentswhether or not explicitly described.

Before describing the various embodiments in more detail, furtherexplanation shall be given regarding certain terms that may be usedthroughout the descriptions.

The term “etch” or “etching” is used herein to generally describe afabrication process of patterning a material, such that at least aportion of the material remains after the etch is completed. Forexample, it should be understood that the process of etching siliconinvolves the steps of patterning a masking layer (e.g., photoresist or ahard mask) above the silicon, and then removing the areas of silicon nolonger protected by the masking layer. As such, the areas of siliconprotected by the mask would remain behind after the etch process iscomplete. However, in another example, etching may also refer to aprocess that does not use a mask, but still leaves behind at least aportion of the material after the etch process is complete.

The above description serves to distinguish the term “etching” from“removing.” When etching a material, at least a portion of the materialremains behind after the process is completed. In contrast, whenremoving a material, substantially all of the material is removed in theprocess. However, in some embodiments, ‘removing’ is considered to be abroad term that may incorporate etching.

During the descriptions herein, various regions of the substrate uponwhich the field-effect devices are fabricated are mentioned. It shouldbe understood that these regions may exist anywhere on the substrate andfurthermore that the regions may not be mutually exclusive. That is, insome embodiments, portions of one or more regions may overlap. Althoughup to three different regions are described herein, it should beunderstood that any number of regions may exist on the substrate and maydesignate areas having certain types of devices or materials. Ingeneral, the regions are used to conveniently describe areas of thesubstrate that include similar devices and should not limit the scope orspirit of the described embodiments.

The terms “deposit” or “dispose” are used herein to describe the act ofapplying a layer of material to the substrate. Such terms are meant todescribe any possible layer-forming technique including, but not limitedto, thermal growth, sputtering, evaporation, chemical vapor deposition,epitaxial growth, electroplating, etc.

The “substrate” as used throughout the descriptions is most commonlythought to be silicon. However, the substrate may also be any of a widearray of semiconductor materials such as germanium, gallium arsenide,indium phosphide, etc. In other embodiments, the substrate may beelectrically non-conductive such as a glass or sapphire wafer.

Before describing such embodiments in more detail, it is instructive topresent an example memory cell and environment in which the presentembodiments may be implemented.

FIG. 1 illustrates an example of a split-gate non-volatile memory cell100. Memory cell 100 is formed on a substrate 102, such as silicon.Substrate 102 is commonly p-type or a p-type well while a first dopedsource/drain region 104 and a second doped source/drain region 106 aren-type. However, it is also possible for substrate 102 to be n-typewhile regions 104 and 106 are p-type.

Memory cell 100 includes two gates, a select gate 108 and a memory gate110. Each gate may be a doped polysilicon layer formed by well known,for example, deposit and etch techniques to define the gate structure.Select gate 108 is disposed over a dielectric layer 112. Memory gate 110is disposed over a charge trapping dielectric 114 having one or moredielectric layers. In one example, charge trapping dielectric 114includes a charge trapping silicon nitride layer sandwiched between twosilicon dioxide layers to create a three-layer stack collectively andcommonly referred to as “ONO.” Other charge trapping dielectrics mayinclude a silicon-rich nitride film, or any film that includes, but isnot limited to, silicon, oxygen, and nitrogen in variousstoichiometries. A vertical dielectric 116 is also disposed betweenselect gate 108 and memory gate 110 for electrical isolation between thetwo gates. In some examples, vertical dielectric 116 and charge trappingdielectric 114 are the same dielectric, while other examples form onedielectric before the other (e.g., they can have different dielectricproperties.) As such, vertical dielectric 116 need not include the samefilm structure as charge trapping dielectric 114. Regions 104 and 106are created by implanting dopants using, for example, an ionimplantation technique. Regions 104 and 106 form the source or drain ofthe split-gate transistor depending on what potentials are applied toeach. In split gate transistors, for convenience, region 104 is commonlyreferred to as the drain, while region 106 is commonly referred to asthe source, independent of the relative biases. It is to be understoodthat this description is meant to provide a general overview of a commonsplit-gate architecture and that, in actual practice, many more detailedsteps and layers are provided to form the final memory cell 100.

An example write, read, and erase operation will now be described as itrelates to memory cell 100. In order to write a bit in memory cell 100,a positive voltage on the order of 5 volts, for example, is applied toregion 106 while region 104 and substrate 102 are grounded. A lowpositive voltage on the order of 1.5 volts, for example, is applied toselect gate 108 while a higher positive voltage on the order of 8 volts,for example, is applied to memory gate 110. As electrons are acceleratedwithin a channel region between the source and drain, some of them willacquire sufficient energy to be injected upwards and get trapped insidecharge trapping dielectric 114. This is known as hot electron injection.In one example of charge trapping dielectric 114, the electrons aretrapped within a nitride layer of charge trapping dielectric 114. Thisnitride layer is also commonly referred to as the charge trapping layer.The trapped charge within charge trapping dielectric 114 store the“high” bit within memory cell 100, even after the various supplyvoltages are removed.

In order to “erase” the stored charge within memory cell 100 and returnthe state of memory cell 100 to a “low” bit, a positive voltage on theorder of 5 volts, for example, is applied to region 106 while region 104is floated or at a certain bias, and select gate 108 and substrate 102are typically grounded. A high negative voltage on the order of −8volts, for example, is applied to memory gate 110. The bias conditionsbetween memory gate 110 and region 106 generate holes throughband-to-band tunneling. The generated holes are sufficiently energizedby the strong electric field under memory gate 110 and are injectedupwards into charge trapping dielectric 114. The injected holeseffectively erase the memory cell 100 to the “low” bit state.

In order to “read” the stored bit of memory cell 100, a low voltage isapplied to each of the select gate, memory gate, and region 104 in therange between zero and 3 volts, for example, while region 106 andsubstrate 102 are typically grounded. The low voltage applied to thememory gate is chosen so that it lies substantially equidistant betweenthe threshold voltage necessary to turn on the transistor when storing a“high” bit and the threshold voltage necessary to turn on the transistorwhen storing a “low” bit in order to clearly distinguish between the twostates. For example, if the application of the low voltage during the“read” operation caused substantial current to flow between regions 104and 106, then the memory cell holds a “low” bit and if the applicationof the low voltage during the “read” operation does not causesubstantial current to flow between regions 104 and 106, then the memorycell holds a “high” bit.

FIG. 2 illustrates an example circuit diagram 200 of memory cell 100including connections to various metal layers in a semiconductor device.Only a single memory cell 100 is illustrated, however, as evidenced bythe ellipses in both the X and Y direction, an array of memory cells maybe connected by the various lines running in both the X and Ydirections. In this way, one or more memory cells 100 may be selectedfor reading, writing, and erasing bits based on the bit line (BL) andsource line (SL) used.

An example source line (SL) runs along the X direction and is formed ina first metal layer (M1). Source line (SL) may be used to makeelectrical connection with doped region 106 of each memory cell 100along a row extending in the X direction.

An example bit line (BL) runs along the Y direction and is formed in asecond metal layer (M2). Bit line (BL) may be used to make electricalconnection with doped region 104 of each memory cell 100 along a columnextending in the Y direction.

It is to be understood that the circuit connections shown in FIG. 2 areonly exemplary and that the various connections could be made indifferent metal layers than those illustrated. Furthermore, although notdepicted, memory cells 100 may be arrayed in the Z direction as wellformed within multiple stacked layers.

FIG. 3 illustrates an example semiconductor device 300 that includesboth memory and peripheral circuitry in the same substrate. In thisexample, substrate 102 includes a core region 302 and a periphery region304. Core region 302 includes a plurality of memory cells 100 that mayoperate similarly to those previously described. It should be understoodthat the cross-section of FIG. 3 is only exemplary, and that core region302 and periphery region 304 may be located in any area of substrate 102and may be made up of various different regions. Furthermore, coreregion 302 and periphery region 304 may exist in the same general areaof substrate 102.

Periphery region 304 may include integrated circuit components such asresistors, capacitors, inductors, etc., as well as transistors. In theillustrated embodiment, periphery region 304 includes a plurality ofhigh-voltage transistors 306 and low-voltage transistors 308. In oneexample, high-voltage transistors 306 exist in a separate region ofsubstrate 102 than low-voltage transistors 308. High-voltage transistors306 are capable of handling voltages up to 20 volts in magnitude, forexample, while low-voltage transistors 308 operate at a faster speed,but cannot operate at the same high voltages as high-voltage transistors306. In an embodiment, low voltage transistors 308 are designed to havea shorter gate length than high voltage transistors 306. High-voltagetransistors 306 are commonly characterized as having a thicker gatedielectric 310 than the gate dielectric of low-voltage transistors 308.

FIGS. 4A-4H illustrate a fabrication process flow for a semiconductordevice including memory cells and other field-effect devices, accordingto an embodiment. It should be understood that the various layers arenot necessarily drawn to scale and that other processing steps may beperformed as well between the steps illustrated as would be understoodby one skilled in the art given the description herein.

FIG. 4A illustrates a cross-section of a semiconductor device 400 thatincludes a substrate 402 having a dielectric layer 404 disposed on top,according to an embodiment. In one example, dielectric layer 404includes a thicker region 406. Thicker region 406 may be used as a gatedielectric for transistors that operate at high voltage magnitudes. Alsodisposed over dielectric layer 404 is a first gate layer 408 followed bya cap layer 410.

In an embodiment, gate layer 408 is a polycrystalline silicon (“poly”)layer. In other examples, gate layer 408 may be any electricallyconductive material such as various metals or metal alloys. Cap layer410, likewise, may include any number of different materials or layerssuch as silicon dioxide or silicon nitride. It is preferable, though notrequired, that cap layer 410 be a material that may be selectivelyremoved.

FIG. 4B illustrates another cross-section of semiconductor device 400after performing an etching process followed by a deposition of a seconddielectric layer 412, according to an embodiment. The etching process isperformed through both cap layer 410 and gate layer 408 down to firstdielectric 404. The etching may use a dry technique such as, forexample, reactive ion etching (RIE) or the etching may use a wettechnique such as, for example, hot acid baths.

The etching is performed to define a select gate 414, according to anembodiment. In this example, select gate 414 may eventually be used asthe select gate for a memory cell as described above with reference toFIG. 1. As such, the etched area illustrated in FIG. 4B may be in amemory cell region (e.g., core region) on substrate 402. The selectgates for two memory cells are illustrated in this example, not intendedto be limiting.

After the etch has been performed, second dielectric layer 412 isdeposited over substrate 402 in at least the memory cell region whereselect gate 414 is formed. In an embodiment, second dielectric layer 412acts as a charge trapping dielectric and includes a specific chargetrapping layer. There are many possible layering structures for thecharge trapping dielectric. In one common example, the charge trappingdielectric is formed by disposing a layer of silicon oxide, followed bya deposition of silicon nitride, followed again by a deposition ofsilicon oxide. This procedure creates what is commonly referred to as an“ONO” stack where the silicon nitride layer sandwiched between the twooxide layers acts as the charge trapping layer. This charge trappinglayer will exist beneath the memory gate and trap charge to set thememory bit as either a ‘0’ or ‘1.’

It should be noted that second dielectric layer 412 is shown as beingdeposited over first dielectric layer 404 in areas between select gates414. However, in another embodiment, first dielectric layer 404 mayfirst be etched away in the exposed areas between select gates 414before second dielectric layer 412 is deposited. Such a procedure mayform a better quality charge trapping dielectric beneath the memorygate.

FIG. 4C illustrates another cross-section of semiconductor device 400after disposing a second gate layer 416 across at least the memory cellregion where select gate 414 is formed. In one embodiment, second gatelayer 416 is a polysilicon layer.

FIG. 4D illustrates the formation of a plurality of memory gates 418,according to an embodiment. Memory gates 418 may be formed via an“etch-back” process, in which a blanket etch is performed across thesubstrate on second gate layer 416. This etch will remove second gatelayer 416 in all areas except those adjacent to the previously definedselect gates 414. As such, memory gates 418 are self-aligned directlyadjacent to both sidewalls of each select gate 414, and are also formeddirectly over second dielectric layer 412. It should also be noted thatin this example, memory gates 418 are taller (e.g., thicker) than selectgates 414. This is due to the existence of cap layer 410 over selectgates 414 while forming memory gates 418.

FIG. 4E illustrates another cross-section of semiconductor device 400where some of the previously patterned memory gates 418 are removed.Each memory cell only requires a single select gate and a single memorygate, according to an embodiment. The removal of the unnecessary gates(e.g., as illustrated at arrows 415) frees up space on substrate 402 inthe memory cell region and also allows for a doped region to beimplanted into substrate 402 aligned adjacent to select gate 414.

Although not indicated in the figure, the source and drain doped regionsare formed in the substrate for each memory cell, according to anembodiment. As mentioned earlier, the drain region is formed adjacent toselect gate 414 in substrate 402, while the source region is formedadjacent to memory gate 418 in substrate 402. In one embodiment, the twomemory cells illustrated may share the same drain region between the twoselect gates 414.

FIG. 4F illustrates another cross-section of semiconductor device 400,according to an embodiment. A first transistor gate 420 is patterned viaan etching process that may be similar to the previous etching processused to define select gates 414. First transistor gate 420 is patternedover thicker region 406 of first dielectric 404. In one embodiment,first transistor gate 420 is the gate for a high-voltage transistordesigned to handle high magnitude voltages. Such voltage magnitudes maybe up to 20 volts. First transistor gate 420 may be formed in a regionon substrate 402 that includes other high-voltage rated devices. Itshould be understood that the single illustrated first transistor gate420 may represent any number of patterned transistor gates over thickerregion 406 of first dielectric 404.

Although second dielectric layer 412 is shown above cap layer 410 in theregion where first transistor gate 420 is patterned, it is not requiredfor this patterning step. In another embodiment, second dielectric layer412 is removed throughout the periphery region of substrate 402 beforeany of the various transistor gates are formed in the periphery region.

After first transistor gate 420 has been patterned, the source and draindoped regions (not shown) are formed adjacent to each side of firsttransistor gate in substrate 402, according to an embodiment. Thejunction depth of each doped region may be deep to accommodate the highmagnitude voltages associated with the field-effect device having firsttransistor gate 420. According to an embodiment, the high ionizationenergy of the dopants to be implanted in substrate 402 is not sufficientto penetrate both the thickness of cap layer 410 and gate layer 408. Ifcap layer 410 was not present during, the implantation process, than thedopants may have been able to penetrate through first transistor gate420, effectively shorting the transistor.

FIG. 4G illustrates another cross-section of semiconductor device 400where cap layer 410 has been removed, according to an embodiment.Optionally, second dielectric layer 412 has also been removed in allareas except between memory gate 418 and substrate 402 and betweenmemory gate 418 and select gate 414. After the removal of cap layer 410,the thickness of first transistor gate is substantially the same as thethickness of gate layer 408. The field-effect device having firsttransistor gate 420 may be formed in a region on substrate 402 separatefrom the memory cell region where a plurality of memory cells 422 areformed.

FIG. 4H illustrates another cross-section of semiconductor device 400where a second transistor gate 424 has been patterned, according to anembodiment. Gate layer 408 is etched to define one or more secondtransistor gates 424 in a region on substrate 402 that may be separatefrom the memory cell region where the plurality of split-gate memorycells 422 are formed. In an embodiment, second transistor gate is formedover first dielectric layer 404, and thus has a thinner gate dielectricthan that associated with first transistor gate 420. In one example,second transistor gate 424 is patterned in a different region onsubstrate 402 than first transistor gate 420. It should be understoodthat the single illustrated second transistor gate 424 may represent anynumber of similarly patterned transistor gates in the same region onsubstrate 402.

After second transistor gate 424 has been patterned, the source anddrain doped regions are formed adjacent to each side of secondtransistor gate in substrate 402. According to an embodiment, the dopedregions associated with second transistor gate 424 are shallower insubstrate 402 than those associated with first transistor gate 420.After second transistor gate 424 has been formed, the thickness ofsecond transistor gate 424 is substantially similar to the thickness offirst transistor gate 420.

At this stage, the various field-effect devices have been formed acrossdifferent regions of substrate 402. As an optional final step, a layerof silicide may be disposed over the various gates and doped areas toincrease the conductivity and reduce parasitic effects such as, forexample, RC delay times for each connection. It should be understoodthat the order and details of each of the fabrication steps illustratedis only exemplary. Some of the processes may be performed in a differentorder or can be combined to create semiconductor device 400 withoutdeviating from the scope or spirit of the invention. For example, thesame etch process may be used to define select gate 414 and firsttransistor gate 420. Other examples may include forming the field effectdevices in the periphery region first (including first transistor gate420 and second transistor gate 424) while forming split-gate memorycells 422 in the memory cell region afterwards. Other similaralterations or deviations may be contemplated by one having skill in therelevant art(s) given the description herein.

FIG. 5 illustrates an example cross-section of a semiconductor device500 having a first transistor 501 and a second transistor 503. In anembodiment, semiconductor device 500 is formed using a substantiallysimilar process as that previous described for forming the peripherytransistors of semiconductor device 400. First transistor 501 includes afirst gate 502 patterned over a thick dielectric layer 508 and alsoincludes doped source/drain regions 510A and 510B. Second transistor 503includes a second gate 504 patterned over a thin dielectric layer 506and also includes doped source/drain regions 512A and 512B.

In an embodiment, first transistor 501 is a high-voltage transistorcapable of handling voltages with magnitudes up to 20 volts. The thickergate dielectric protects first transistor 501 from dielectric break-downwhen applying the high voltage magnitudes. Furthermore, doped regions510A and 510B are implanted deep into substrate 402 to accommodate thelarger depletion regions and electric fields that are generated.

In an embodiment, second transistor 503 is a low-voltage transistordesigned for fast switching speeds and capable of handling lower voltagemagnitudes up to about 5 volts. Having a short gate length (L1)decreases the switching speed of second transistor 503. In anembodiment, the gate length (L1) of second transistor 503 is less thanor equal to half the gate length (L2) of first transistor 501. In onesuch example, L1 is 45 nm while L2 is at least 90 nm. In anotherexample, L1 is between 10-40 nm. The junction depths of doped regions510A and 510B are also deeper than the junction depths of 512A and 512B,according to an embodiment. However, even with the difference in gatelengths and junctions depths between first transistor 501 and secondtransistor 503, the thickness of first gate 502 is substantially equalto the thickness of second gate 504. Any discrepancy caused by the extrathickness afforded due to thicker dielectric layer 508 is considerednegligible.

The fabrication process flow illustrated in FIGS. 4A-4H demonstrate anexample where the various split-gate memory cells are formed with theselect gate being defined first, followed by the memory gateself-aligned to a sidewall of the select gate. Such a process ultimatelyresults in the memory gate being thicker than the select gate as shownin FIG. 4H, according to one embodiment. However, the invention is notlimited to forming the select gate before the memory gate, and inanother embodiment, the memory gate is formed first followed by aself-aligned select gate. An example process flow for forming the memorygate first is illustrated in FIGS. 6A-6F. Note that FIGS. 6A-6F onlyillustrate cross-sections of the memory cell region and thus do notillustrate the formation of the various transistors in the other (e.g.,periphery) regions.

FIG. 6A illustrates a cross-section of a semiconductor device 600 thatincludes a substrate 602 having a charge trapping dielectric 604disposed on top, according to an embodiment. Also disposed over chargetrapping dielectric 604 is a gate layer 606 followed by a cap layer 608.Cap layer 608 and first gate layer 606 may be substantially similar togate layer 408 and cap layer 410 as described previously with referenceto FIGS. 4A-4H.

Charge trapping dielectric 604 may be similar to the previouslydescribed second dielectric layer 412. As such, charge trappingdielectric 604 may be a “ONO” stack according to one embodiment.

FIG. 6B illustrates another cross-section of semiconductor device 600where a plurality of memory gates 610 have been formed, according to anembodiment. Memory gates 610 are defined in a similar manner aspreviously described for creating select gates 414.

Charge trapping dielectric 604 has been etched such that portions existonly beneath memory gates 610. Afterwards, a second dielectric layer 612is disposed over substrate 602. Second dielectric layer 612 may besilicon dioxide, and also covers the sidewalls of memory gates 410during the deposition process.

FIG. 6C illustrates another cross-section of semiconductor device 600where a second gate layer 614 has been disposed. In one embodiment,second gate layer 614 is a polysilicon layer.

FIG. 6D illustrates another cross-section of semiconductor device 600where a plurality of select gates 616 are formed adjacent to bothsidewalls of each memory gate 610. Select gates 616 may be formed usinga similar “etch-back” process as described previously for forming memorygates 418 with reference to FIG. 4D. It should also be noted that inthis example, select gates 616 are taller (e.g., thicker) than memorygates 610. This is due to the existence of cap layer 608 over memorygates 610 while forming select gates 616.

FIG. 6E illustrates another cross-section of semiconductor device 600where some of the previously patterned select gates 616 are removed.Each memory cell only requires a single select gate and a single memorygate, according to an embodiment. The removal of the unnecessary gatesfrees up space on substrate 602 in the memory cell region and alsoallows for a doped region to be implanted into substrate 602 alignedadjacent to each memory gate 610.

FIG. 6F illustrates another cross-section of semiconductor device 600where cap layer 608 has been removed and the formation of a plurality ofsplit-cell memory cells 618 is nearly complete, according to anembodiment. Optionally, second dielectric layer 612 has been etched awayin all areas except under select gates 616 and between select gates 616and memory gates 610.

Although not indicated in the figure, after the final patterning ofselect gates 616 and/or removal of cap layer 608, the source and draindoped regions are formed in the substrate for each split-gate memorycell, according to an embodiment. As mentioned earlier, the drain regionis formed adjacent to select gates 616 in substrate 602 while the sourceregion is formed adjacent to memory gates 610 in substrate 602. In oneembodiment, the two memory cells illustrated may share the same drainregion between two select gates 616.

Once split-gate memory cells 618 have been fully formed up through FIG.6E, other transistors may be formed in the periphery region in a similarmanner as described earlier with reference to FIGS. 4F-4H. Also, similarto the process flow illustrated for semiconductor device 400, the stepsillustrated in FIGS. 6A-6F is only one example for forming semiconductordevice 600. The steps may be performed in a different order or may becombined in some aspects to generate a substantially similar finalstructure. Such modifications would be apparent to one having skill inthe relevant art(s) given the description herein.

It is to be appreciated that the Detailed Description section, and notthe Summary and Abstract sections, is intended to be used to interpretthe claims. The Summary and Abstract sections may set forth one or morebut not all exemplary embodiments of the present invention ascontemplated by the inventor(s), and thus, are not intended to limit thepresent invention and the appended claims in any way.

The present invention has been described above with the aid offunctional building blocks illustrating the implementation of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have been arbitrarily defined herein for the convenienceof the description. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

What is claimed is:
 1. A method of fabricating a split gate memory device, comprising: forming a first dielectric layer and a memory gate layer over at least a first region of a substrate; forming a cap layer over the memory gate layer; patterning the cap, memory gate, and first dielectric layers to form a memory gate stack forming a second dielectric layer over the memory gate stack; forming a select gate layer overlying the second dielectric layer; patterning the select gate layer such that only a single select gate stack is formed adjacent to the memory gate stack; forming doped regions within the substrate adjacent to the memory gate stack and the select gate stack, respectively; and removing the cap layer from the memory gate stack, wherein forming the select gate stack comprises forming the select gate stack to comprise a planar top surface parallel to a surface of the substrate.
 2. The method of claim 1, wherein forming the first dielectric layer includes disposing a charge trapping layer overlying the substrate.
 3. The method of claim 1, wherein patterning the cap, memory gate, and first dielectric layers includes performing an etching process to remove the cap, memory gate, and first dielectric layers beyond the memory gate stack.
 4. The method of claim 3, wherein the etching process includes at least one of reactive ion etching, hot acid baths, a dry etching process, or a wet etching process.
 5. The method of claim 1, wherein forming the second dielectric layer includes disposing the second dielectric layer conformably over two side surfaces and a top surface of the memory gate stack.
 6. The method of claim 1, wherein patterning the select gate layer includes: performing an etch-back process on the select gate layer across the substrate such that the select gate layer is removed in all areas except in areas adjacent to the memory gate stack.
 7. The method of claim 5, wherein the select gate layer patterned is self-aligned adjacent to the side surfaces of the memory gate stack, and further comprising: removing the select gate layer patterned adjacent to one of the side surfaces of the memory gate stack.
 8. The method of claim 1, wherein forming the doped regions includes forming a drain region adjacent to the select gate stack and a drain region adjacent to the memory gate stack.
 9. The method of claim 1, wherein a top surface of the select gate stack has a higher elevation than the top surface of the memory gate stack after the cap layer is removed.
 10. The method of claim 1, wherein the cap layer is selectively removable and configured to control a height of the memory gate layer in the memory gate stack.
 11. The method of claim 1, wherein a vertical portion of the second dielectric layer is disposed between the memory and select gate stacks.
 12. The method of claim 1, further comprising: forming a low-voltage transistor including a first gate layer disposed over a low-voltage dielectric layer in a second region of the substrate; and forming a high-voltage transistor including the first gate layer disposed over a high-voltage dielectric layer in a third region of the substrate, wherein the high-voltage dielectric layer is thicker than the low-voltage dielectric layer.
 13. The method of claim 1, wherein forming the memory gate stack comprises forming the memory gate stack to comprise a planar top surface parallel to a surface of the substrate. 